The present invention relates to acidic etching compositions and methods useful for texturing the surface of silicon substrates, particularly for polycrystalline, or multicrystalline silicon wafers used in manufacturing solar cells.
Renewable energy sources are considered to be green or clean energy sources having a much lower environmental impact than conventional fossil based energy sources. Of the renewable sources, solar energy is the most abundant energy resource on earth. Photovoltaic energy in particular is becoming a more reliable and readily available source of generating electricity. Currently the most used material for the manufacturing of solar cells is crystalline silicon.
Optical losses due to the reflectance of incident solar radiation are one of the most significant factors that limit solar efficiency. Lowering the surface reflection of silicon wafers by texturization is practiced in order to improve a solar cell's efficiency. Increased light absorption from a better-textured silicon surface will result in higher currents from the cell, which can result in higher efficiency of the solar cell.
Texturing is a means of creating a certain roughness at the surface of a substrate in order to enable multiple reflection of light incident on its surface, thereby leading to greater absorption of the light inside the substrate, i.e., to an increased light-confining effect. The roughness obtained in this way has two complementary effects: the first effect is to reduce the reflecting power or optical reflectivity of the surface; the second effect is to increase the length of the optical path travelled by the incident light inside the substrate. In a photocell, photovoltaic cell or solar cell, the increased light-confining effect gives rise to an increase in the effectiveness with which light is transformed into electricity.
Customarily, the roughness or texture of the surface of a substrate, in particular of a silicon or silicon alloy substrate or wafer may include randomized pyramids or a multitude of pits having a depth in the range of from 0.1 to 10 μm and a diameter in the range of from 0.1 to 10 μm.
Various methods have been proposed for texturing the surface of the substrates, including mechanical engraving, laser etching, photolithography, masking, sandblasting, mortizing, anodic oxidation, sputter etching, etc. However, these methods are complex and expensive to implement.
The single crystal silicon substrates are mainly produced by sawing massive silicon ingots. However, crystal defects having a depth of several μm are caused by the sawing, which is known in the art as saw damage. Since these crystal defects are centers for the recombination of electron-hole pairs, it is necessary to remove them by way of so-called saw damage etch.
The method of surface texturing presently employed on an industrial scale includes the use of aqueous alkaline solutions based on sodium hydroxide or potassium hydroxide. These solutions have the property of etching silicon anisotropically depending on the crystallographic orientation of the grains situated at the surface, thereby modifying the surface morphology of the silicon. Etching speed is about 100 times greater on planes having a certain crystallographic orientation than on other planes. This causes the surface to be textured in the form of randomized pyramids consisting of the most stable Si(111) plane, that trap the incident light, which effect is referred to as “micro-texturing.” For polycrystalline silicon substrates however, it is estimated that only 20% of the surface is constituted by grains having the preferred crystallographic orientation to result in pyramids. The treatment is therefore less effective with respect to optical reflectivity. This equally applies to silicon substrates manufactured by the edge defined film-fed growth (EFG) process or by the string ribbon process, which processes do not require sawing. Thus, the treatment is fully effective only in the special case of single crystal silicon substrates having a single crystallographic orientation at the surface to be etched.
In order to ameliorate these problems and drawbacks, aqueous acidic etching solutions and methods of their use have been developed.
A typical acidic process used in industry is known as the UKON etch, and consists of a mixture of hydrofluoric acid (HF), nitric acid (HNO3), and water (H2O) in varying amounts. See, for example, U.S. Pat. No. 5,949,123 and EP 0 773 590 B1 which disclose an aqueous acidic etching solution containing hydrofluoric acid and nitric acid in a weight ratio of 1:3. This etching solution is used for creating pores in the silicon surface resulting in a porous layer which still contains some amorphic silicon. In a second step, this amorphic silicon is removed with an alkaline solution of sodium hydroxide, thereby creating a surface texture.
U.S. Pat. No. 6,340,640 and German patent application DE 197 46 706 A1 disclose an acidic etching solution containing 12 parts by volume of 50% by weight hydrofluoric acid, one part by volume of 69% by weight nitric acid and 12 parts by volume of 85% by weight phosphoric acid. Instead of the phosphoric acid, a carboxylic acid having a higher molecular weight than acetic acid can be used. Additionally, the acidic etching solution can also contain surfactants or ammonium fluoride.
International patent application WO 01/47032 A1 discloses acidic etching solutions containing 1 to 30% by weight of hydrofluoric acid, 5 to 30% by weight of nitric acid and 50 to 94% by weight of sulfuric acid and/or phosphoric acid. The preferred composition contains 10 to 16% by weight hydrofluoric acid, 15 to 25% by weight nitric acid, 15 to 25% by weight sulfuric acid, 14 to 20% by weight phosphoric acid, and 20 to 30% by weight water. An even more preferred composition contains 3 to 7% by weight hydrofluoric acid, 3 to 7% by weight nitric acid, 75 to 85% by weight sulfuric acid, and 5 to 15% by weight water.
Japanese patent application JP 2004-063744 discloses a two-step etching process, wherein an aqueous acidic etching solution A, containing 36 to 42% by weight hydrofluoric acid and 6 to 10% by weight nitric acid, and an aqueous acidic etching solution B, containing 32 to 46% by weight hydrofluoric acid and 2 to 6% by weight nitric acid, are used.
U.S. Pat. No. 7,192,885 (international patent application WO 2004/100244 A1) discloses an aqueous acidic etching solution containing 10 to 40% by weight concentrated hydrofluoric acid, 2 to 60% by weight concentrated nitric acid, and 20 to 55% by weight water.
Japanese patent application JP 2005-31 1060 discloses an aqueous acidic etching solution containing 10 to 22% by weight hydrofluoric acid, 15 to 31% by weight nitric acid, the remainder being water.
German patent application DE 10 2007 004 060 A1 discloses an aqueous acidic solution containing 0.8 to 2% by weight hydrofluoric acid, 15 to 40% by weight nitric acid, 10 to 41% by weight sulfuric acid, the remainder being 74.2% by weight or less of water. A preferred solution contains 1 to 1.7% by weight hydrofluoric acid, 20 to 30% by weight nitric acid, 18 to 35% by weight sulfuric acid, and 61% by weight or less water. A particularly preferred solution contains 1.4% by weight hydrofluoric acid, 27% by weight nitric acid, 26% by weight sulfuric acid, and 45.6% by weight water. However, this solution is primarily used for the wet chemical edge isolation and, therefore, is used to solve a technical problem which is different from the problems of the texturing of surfaces. If it is used for this purpose, it exhibits only little etching power and leads to comparatively smooth surfaces. Additionally, several commercial aqueous acidic etching solutions are available in the market.
International application WO2011/032880 also discloses an aqueous acidic etching solution for texturing both single crystal and polycrystalline silicon wafers. The etching solution includes 3 to 10% by weight hydrofluoric acid, 10 to 35% by weight nitric acid, 5 to 40% by weight sulfuric acid, and 55 to 82% by weight water.
Products used in such etching includes, for example, Spin-Etch™ BT which contains 10 to 20% by weight hydrofluoric acid, 20 to 30% by weight nitric acid, 15 to 25% by weight sulfuric acid and 10 to 20% by weight phosphoric acid, the remainder being 35% by weight or less water. Si-Polish-Etch™ I contains 5 to 10% by weight hydrofluoric acid, 36 to 38% by weight nitric acid, 9 to 13% by weight sulfuric acid and 16 to 20% by weight phosphoric acid, the remainder being 34% by weight or less water. Spin-Etch™ D contains 1 to 7% by weight hydrofluoric acid, 35 to 45% by weight nitric acid, 10 to 20% by weight sulfuric acid and 15 to 25% by weight phosphoric acid, the remainder being 39% by weight or less water. And, Spin-Etch™ E contains 1 to 5% by weight hydrofluoric acid, 5 to 10% by weight nitric acid and 75 to 85% by weight sulfuric acid, the remainder being 19% by weight or less water.
Although these prior art aqueous acidic etching solutions are capable of ameliorating some of the problems and drawbacks associated with the aqueous alkaline etching solutions, they require constant development to improve and balance their application property profile in order to meet the demands of the market. More particularly, it is desirable to produce a silicon wafer which contributes to increased efficiency of the solar cell by being less reflective of solar rays at its surface.
Additionally, the prior art compositions and processes result in wafers with significant defect etching at grain boundaries or at areas of poorer quality on the wafer. As a result of the defects on the wafer surface, undesired charge recombination increases. Moreover, the wafers become more brittle because of the deeper etch at defects increasing the chance for breakage. Additionally, the desired texture is obtained in a very narrow window of the HF/HNO3/H2O phase diagram which makes it difficult to control. Ideally, a texturing solution would be capable of etching off a higher amount of silicon from the wafer, whilst displaying equal or lesser amounts of defect etching. This entails removing more of the saw damage, which results in charge recombination, and avoiding problems associated with current known products, such as enhanced defect etching at deeper etch depths.
Still further with regard to additives to the UKON etch process, additional problems have arisen, such as increased foaming of the mixture and surfactants sticking to the wafers. All of these problems lead to increased contamination of the wafers, requiring, e.g., dragging in other baths and extra cleaning steps. Also, some additives, particularly fluorinated alkyl chains, present additional environmental concerns.
Because of the physical/structural differences among the number of different types of silicon wafers from varied sources (e.g., REC Singapore, REC Norway, SolarWorld, IMEC, and ECN), one acidic bath does not fit all. It is important to provide a system and compositions which can be adapted to provide an efficient texturing process for each of the varied types of polycrystalline silicon wafers.
There remains, therefore, a need for improved chemical texturing methods and compositions for polycrystalline silicon wafers, particularly those used in the manufacturing of solar cells in the photovoltaic industry.